1. Field of the Invention
The present invention relates to a solid-state image sensing device, an analog-digital conversion method of the solid-state image sensing device, and an electronic apparatus. Specifically, an X-Y address-type solid-state image sensing device represented by a CMOS (including MOS) image sensor, an analog-digital conversion method, and an electronic apparatus.
2. Description of the Related Art
An imaging apparatus such as a digital still camera that converts light into electric signals and outputs image signals uses a solid-state image sensing device as its image capture unit (photoelectric conversion unit). In the field of the solid-state image sensing device, recently, with the larger number of pixels and the higher frame rate, technologies of realizing faster readout and lower power consumption have been indispensable.
As a kind of solid-state image sensing device, there is a CMOS (including MOS)-type image sensor (hereinafter, referred to as “CMOS image sensor”) having a feature that the sensor may be manufactured in the same process as that of a CMOS integrated circuit. The CMOS image sensor has a configuration that converts charge into an electric signal with respect to each pixel and processes the electric signals read out from the pixels in parallel with respect to each pixel column. The parallel processing with respect to each pixel column can improve the readout speed of the pixel signals.
In related art, as a CMOS image sensor that reads out signals from plural pixels arranged in a matrix in parallel with respect to each pixel column, one using a column AD conversion system that performs analog-digital conversion (hereinafter, referred to as “AD conversion”) on pixel signals with respect to each pixel column is known (for example, see JP-A-2005-278135).
The column AD conversion system CMOS image sensor has a configuration that shares signal readout lines in the vertical direction of the pixels two-dimensionally arranged in a matrix (hereinafter, referred to as “vertical signal lines”), and includes an AD conversion circuit and a readout circuit with respect to each pixel column. Further, the sensor performs simultaneous signal processing worth the total number of the pixel columns by driving the AD conversion circuits and the readout circuits at the same time.
The AD conversion circuit performs AD conversion by the following series of circuit operation. That is, first, the circuit compares the analog pixel signal read out from the pixel through the vertical signal line to the reference signal with a slope waveform having a certain gradient and linearly changing with respect to each pixel column using a comparator, and, at the same time, starts a count operation of a counter. In this regard, the counter performs the count operation in synchronization with clocks having a fixed period.
Then, the AD conversion circuit stops the count operation of the counter with inversion timing of the output of the comparator when the analog pixel signal and the reference signal intersect. The final count value of the counter is a digital signal according to the magnitude of the analog pixel signal. As described above, the column AD conversion system is a readout system characterized by high-speed imaging for AD conversion of pixel signals for one row at a time.